In automotive vehicles it is known to provide a hydrostatic transmission between an internal-combustion engine and a load, e.g. the wheels of a vehicle, in which the hydrostatic transmission has a control element which can be shifted to either of two opposite directions from a neutral position so as to cause operation of the output shaft of the transmission in either of two opposite directions selectively.
The control element of this transmission, usually the swashplate or stroke or displacement controller of the axial piston pump of the transmission is usually displaced by a servomechanism which can include a three-position valve, the position of which is directly or indirectly selected by a direction control member, e.g. a lever. The valve, in turn, operates a servocylinder which follows the actuation of the valve and has a piston connected to the control element of the axial piston pump. The servomechanism is operated to control pressure from a control pump or some other source and a brake pedal is provided to control a throttle which selectively reduces the control pressure to permit a return of the control element of the axial piston pump to its neutral position for hydrodynamic braking via the transmission.
Such a system is described in the publication DE-AS No. 1,555,480. In this case, the control braking of the device is effected either by reducing the supply of fuel to the engine and thereby reducing the engine speed (engine braking) or by actuating the brake pedal to open the aforementioned throttle valve and reduce the control pressure for the servomechanism.
The pressure drop in the supply to the servomechanism results in a restoration of the hydrostatic pump control member to its neutral position.
It is important in practice to ensure, especially in forklift trucks and like vehicles, that the control element of the hydrostatic transmission has been brought fully to neutral and the vehicle to standstill before a direction change. In the case of a forklift truck this is especially important to prevent undesired shifting of the load, damage of the vehicle and injury to personnel in the region thereof. On the other hand, it is desirable to allow the operator of the vehicle to shift his control lever from one directional position into the opposite directional position in spite of the fact that full standstill has not been reached and the deceleration process is yet in progress. This allows the operator of the vehicle to concentrate upon other control processes and pay attention to other requirements which might be overlooked if, in addition to controlling the braking operation, he must select the direction for future movement of the vehicle.